Regulations in the United States and other nations place limits on the levels of pollution that can be emitted into the air or water by various processes. For example, the regulations limit the opacity, particulate matter content, and other properties of stack gas emitted by electric utilities, industrial and other sources. Many of these regulations require that outgoing stack gas be monitored to ensure compliance with the standards. Accordingly, instruments have been developed to monitor outgoing stack gas and other fluids.
Various existing instruments operate by directing a beam of optical energy across a stack or other fluid containing area and measuring the portion of the beam that is extinguished, scattered, or otherwise interacts with the fluid. One such device is a double-pass opacity monitor. A double-pass opacity monitor, such as opacity monitor 100 shown in FIG. 1, includes a transceiver assembly 102 on a first side of a fluid 101 and a reflector assembly 104 on a second side of the fluid 101 opposite the transceiver 102. The fluid 101 may be contained within a stack (not shown) positioned between the transceiver assembly 102 and reflector assembly 104. In use, a light source 108 of the transceiver assembly 102 emits a forward beam 120 of optical energy that is reflected by beam splitter 110 toward an aperture 106. An imaging lens 105 is present at or near the aperture 106 and directs the forward beam 120 out of a purge nozzle 114 of the transceiver assembly 102 and toward the reflector assembly 104. At the reflector assembly 104, the beam 120 is received by a purge nozzle 116 where it may be incident on corner cube reflector 112. The reflector 112 reflects a portion of the forward beam 120 back toward the transceiver assembly 102 as a reverse beam 122. The reverse beam 122 is incident on the aperture 106 and imaging lens 105, which focus the reverse beam 122 through the beam splitter 110 and onto a sensor 109. The difference between the intensity of the return beam 122 in a clear environment and the intensity of the return beam 122 when a fluid, such as stack gas 101, is present yields an indication of the opacity of the fluid 101.
FIG. 1 shows a typical prior-art configuration of the opacity monitor 100. As shown, the transceiver assembly 102 directs a diverging forward beam 120 across the stack, represented by the distance d, and toward the reflector assembly 104, where the beam 120 over-fills the reflector 112. That is, when the forward beam 120 is incident on the reflector 112, it has diverged to a diameter greater than that of the reflector 112. This wastes of a portion of the optical energy of the beam 120, but greatly simplifies the process of aligning the transceiver assembly 102 and reflector assembly 104. The return beam 122 then traverses the distance d again, until it is incident on the aperture 106 and lens 105. The diameter of the reflector 112 is selected to configure the return beam 122 to approximately fill the aperture 106. It is not desirable to significantly over-fill the aperture 106 because this wastes additional optical energy. Also, in prior designs, it was thought undesirable to significantly under-fill the aperture 106 because this will reduce the intensity of the return beam, and consequently the signal-to-noise ratio of the monitor 100.
Although the opacity monitor 100 shown by FIG. 1 should theoretically produce accurate results, in practice, it does not. Opacity monitors, such as monitor 100, and other similar optical instruments, have long exhibited an unexplained negative bias when installed on a stack. That is, the instruments often produce an opacity reading that is lower than the actual opacity of the fluid 101. It is believed that this negative bias affects results at all observed opacity levels, however, it is most apparent in relatively clean stacks with relatively low opacity. In some of these stacks, the actual opacity of the stack gas is less than the amount of the negative bias, causing the instrument to read a negative opacity. Predictably, sources that report negative opacity have been subjected to scrutiny and accused of having malfunctioning monitors. Also, environmental regulatory agencies are believed to have penalized sources that report excessive negative opacity readings. Paradoxically, as sources have improved process control and installed more effective pollution control devices, their stacks have become cleaner and therefore more likely to exhibit a negative opacity reading as a result of the negative bias. There has been much speculation about the source of the negative bias, but there are still no satisfactory ways of dealing with it.